Introduction

RFID technology is an exciting technology. The EM4095 chip allows us to create a reader with little code or processor resources.

A complete KIT is available from the web shop at www.mcselec.com

 

This topic describes the reference design.

The data sheets you can download from:

 

EM4095 (chip)  , EM4102  (transponder)

 

 

The circuit

 

As you can see from the data sheets, the EM4095 needs little external hardware. A coil, capacitors that tune the coil for 125 KHz, are basically all that you need. IC1 is a voltage regulator that regulates the input voltage to 5V. (you can operate it from a 9V battery). The capacitors stabilize the output voltage. The DEMOD output of the EM4095 is connected to the microprocessor and the pin is used in input mode. The MOD and SHD pins are connected to micro pins that are used in output mode.

 

The micro(mega88) has a small 32 KHz crystal so the soft clock can be used. There are 3 switches that can be used for menu input, and there is a relay that can be used to activate a door opener. Parallel on the relay there is a LED for a visible indication.

IC4 is a serial interface buffer so we can connect the PCB to our computer for logging and programming. The Mega88 is delivered with a Boot loader and thus can be serial programmed  with the MCS Boot loader. That is why pin 4 of X6 (DTR) is connected via IC4(pin 8-9) to the reset pin of the micro(pin 1).

Further there is a standard 10-pins ISP programmer connector for the USB-ISP or STK200, and an LCD connector for an optional LCD display.

 

 

The PCB

 

 

Part list

 

 

Building the PCB

As usually we start with the components that have the lowest height.  And normally we would solder all passive components first, and insert/solder the active components last. This to prevent damage to the active components(IC). But since the EM4095 is only available in SMD, we need to solder this chip first. Make sure the chip is lined out right and that pin 1 matches the small dot on the chip which is an indication for pin 1.

Then solder pin 1 and 16 so the chip can not be moved anymore. Now solder the remaining pins. Use an iron with a small tip. When you use too much solder, and two feet are soldered together do not panic. Just finish soldering and when ready, use some copper braid to remove the solder between the 2 feet. This works best when you lay the braid over the 2 pins, then push the solder iron to the braid so it will heat up. Then after some seconds, add some solder which will get sucked into the braid. This will in turn suck the other solder into the braid. While it does not seem logical to add solder, it will conduct the heat better. But since the used SMD chip is relatively large there should not be any problem.

Now mount and solder the following components :

 

Operation

Now the PCB is ready. Make sure there are no solder drops on the PCB. You can measure with an Ohm-meter if there is a short circuit.

Measure pin 1 and pin 2 of IC1 (the voltage input) and pin 3 and pin 2 of IC1 (the voltage output).

When everything is ok, insert the MAX232 and the MEGA88.

You can connect the battery cord to header X1. The red wire is the plus. Since the circuit is not for beginners, there is no reverse polarity protection. While the 7805 does not mind a short circuit, the C1 elco might not like it.

Connect the battery and measure with a Volt meter if IC1 actual outputs 5V. If not, check the input voltage, and for a possible shortcut.

 

Connect the antenna to connector X2. The PCB is now ready for use. When you have the LCD display, connect it to the LCD header and adjust the variable resistor R9 so you can see square blocks.

 

Since the chip has a boot loader, you can serial program the device. We made a simple AN that can be used as a door opener. It has simple menu, and we can add new tags. When a valid tag is held in front of the antenna, it will activate the relay for 2 seconds. The LED will be turned on as well.

Compile the program AN_READHITAG_EM4095.BAS and select the MCS Boot Loader programmer. Connect a serial cable to X6 and press F4 to program.

 

You need a normal straight cable.

 

When you did not used the MCS Bootloader before, check the COM port settings and make sure the BAUD is set to 38400 as in the following screenshot:

 

 

You also need to set 'RESET via DTR' on the 'MCS Loader' TAB.

Now the program will start and show some info on the LCD. Each time you hold a RFID tag before the antenna/coil, the TAG ID will be shown.

When you press S3, you can store an RFID. Press S3, and then hold the TAG before the coil. When there is room , or the tag is new, it will be stored. Otherwise it will be ignored. The TAG ID is also stored in EEPROM.

Now when you hold the tag before the coil, the relay is activated for 2 seconds.

The AN is very simple and you can change and extend it easily.

One nice idea from Gerhard : use one TAG as a master tag to be able to add/remove tags.

 

Security

To make the code more secure you could add a delay so that a valid tag must be received twice, so after the valid TAG, wait 1 second, and then start a new measurement and check if the TAG is valid again.

This will prevent where a bit generator could be used to generate all possible codes. With 64 bit times a second, it  would take ages before it would work.

The other hack would be to listen with a long range 125 KHz antenna, and recording all bits. A long range scanner would be very hard to make. It would be easier to open the door with a crowbar.

When you open your door with this device, make sure you have a backup option like a key in case there is no power. Also, when the door is opened by a magnetic door opener, make sure it has the right quality for the entrance you want to protect.  

 

 

 

AN Code

'-------------------------------------------------------------------------------

'                  (c) 1995-2021 MCS Electronics

'  This sample will read a HITAG chip based on the EM4095 chip

'  Consult EM4102 and EM4095 datasheets for more info

'-------------------------------------------------------------------------------

'  The EM4095 was implemented after an idea of Gerhard Günzel

'  Gerhard provided the hardware and did research at the coil and capacitors.

'  The EM4095 is much simpler to use than the HTRC110. It need less pins.

'  A reference design with all parts is available from MCS

'-------------------------------------------------------------------------------

$regfile = "M88def.dat"

$baud = 19200

$crystal = 8000000

$hwstack = 40

$swstack = 40

$framesize = 40

 

 

Declare Function Havetag(b As Byte ) As Byte

 

'Make SHD and MOD low

_md Alias Portd.4

Config _md = Output

_md = 0

 

_shd Alias Portd.5

Config _shd = Output

_shd = 0

 

Relay Alias Portd.2

Config Relay = Output

 

S3 Alias Pinb.0

S2 Alias Pinb.2

S1 Alias Pinb.1

Portb = &B111                                               ' these are all input pins and we activate the pull up resistor

 

Config Clock = Soft                                         'we use a clock

Config Date = Dmy , Separator = -

Enable Interrupts                                           ' the clock and RFID code need the int

Date$ = "15-12-07"                                          ' just a special date to start with

Time$ = "00:00:00"

 

'Config Lcd Sets The Portpins Of The Lcd

Config Lcdpin = Pin , Db4 = Portc.2 , Db5 = Portc.3 , Db6 = Portc.4 , Db7 = Portc.5 , E = Portc.1 , Rs = Portc.0

Config Lcd = 16 * 2                                         '16*2 type LCD screen

Cls

             Lcd " EM4095 sample"

Lowerline : Lcd "MCS Electronics"

 

Dim Tags(5) As Byte                                         'make sure the array is at least 5 bytes

Dim J As Byte , Idx As Byte

Dim Eramdum As Eram Byte                                    ' do not use first position

Dim Etagcount As Eram Byte                                  ' number of stored tags

Dim Etags(100) As Eram Byte                                 'room for 20 tags

Dim Stags(100) As Byte                                      'since we have enough SRAM store them in sram too

Dim Btags As Byte , Tmp1 As Byte , Tmp2 As Byte

Dim K As Byte , Tel As Byte , M As Byte

 

Config Hitag = 64 , Type = Em4095 , Demod = Pind.3 , Int = @int1

Print "EM4095 sample"

 

 

'you could use the PCINT option too, but you must mask all pins out so it will only respond to our pin

' Pcmsk2 = &B0000_0100

' On Pcint2 Checkints

' Enable Pcint2

On Int1 Checkints Nosave                                    'we use the INT1 pin all regs are saved in the lib

Config Int1 = Change                                        'we have to config so that on each pin change the routine will be called

Enable Interrupts                                           'as last we have to enable all interrupts

 

 

'read eeprom and store in sram

'when the program starts we read the EEPROM and store it in SRAM

For Idx = 1 To 100                                          'for all stored tags

   Stags(idx) = Etags(idx)

   Print Hex(stags(idx)) ; ",";

Next

 

Btags = Etagcount                                           ' get number of stored tags

If Btags = 255 Then                                         ' an empty cell is  FF (255)

   Print "No tags stored yet"

   Btags = 0 : Etagcount = Btags                            ' reset and write to eeprom

Else                                                        ' we have some tags

   For J = 1 To Btags

       Tmp2 = J * 5                                         'end

       Tmp1 = Tmp2 - 4                                      'start

       Print "RFID ; " ; J                                  ' just for debug

       For Idx = Tmp1 To Tmp2

         Print Hex(stags(idx)) ; ",";

       Next

       Print

   Next

End If

 

Do

   Print "Check..."

   Upperline : Lcd Time$ ; " Detect"

   If Readhitag(tags(1)) = 1 Then                           'this will enable INT1

      Lowerline

      For J = 1 To 5

         Print Hex(tags(j)) ; ",";

         Lcd Hex(tags(j)) ; ","

      Next

      M = Havetag(tags(1))                                  'check if we have this tag already

      If M > 0 Then

         Print "Valid TAG ;" ; M

         Relay = 1                                          'turn on relay

         Waitms 2000                                        'wait 2 secs

         Relay = 0                                          'relay off

      End If

      Print

  Else

     Print "Nothing"

  End If

  If S3 = 0 Then                                            'user pressed button 3

     Print "Button 3"

     Cls : Lcd "Add RFID"

     Do

       If Readhitag(tags(1)) = 1 Then                       'this will enable INT1

          If Havetag(tags(1)) = 0 Then                      'we do not have it yet

             If Btags < 20 Then                                'will it fit?

                Incr Btags                                     'add one

                Etagcount = Btags

                Idx = Btags * 5                                'offset

                Idx = Idx - 4

                Lowerline

                For J = 1 To 5

                  Lcd Hex(tags(j)) ; ","

                  Stags(idx) = Tags(j)

                  Etags(idx) = Tags(j)

                  Incr Idx

                Next

                Cls

                Lcd "TAG stored" : Waitms 1000

             End If

          End If

          Exit Do

       End If

     Loop

  End If

  If S2 = 0 Then

     Print "Button 2"

  End If

  If S1 = 0 Then

     Print "Button 1"

  End If

 

  Waitms 500

Loop

 

 

 

'check to see if a tag is stored already

'return 0 if not stored

'return value 1-20 if stored

Function Havetag(b As Byte ) As Byte

  Print "Check if we have TAG : ";

  For K = 1 To 5

     Print Hex(b(k)) ; ","

  Next

 

 

  For K = 1 To 20

    Tmp2 = K * 5                                            'end addres

    Tmp1 = Tmp2 - 4                                         'start

    Tel = 0

    For Idx = Tmp1 To Tmp2

       Incr Tel

       If Stags(idx) <> B(tel) Then                         'if they do not match

          Exit For                                          'exit and try next

       End If

    Next

 

    If Tel = 5 Then                                         'if we did found 5 matching bytes we have a match

       Print "We have one"

       Havetag = K                                          'set index

       Exit Function

    End If

  Next

  Havetag = 0                                               'assume we have nothing yet

 

End Function

 

 

 

Checkints:

 Call _checkhitag                                           'in case you have used a PCINT, you could have other code here as well

Return

 

 

Tips and Tricks

The oscillator frequency must be 125 KHz. You can measure this with an oscilloscope. It is possible that you need to remove a few windings of the antenna coil to get an exact 125 KHz. This will result in a higher distance that you can use for the tags.